Anybody notice better mileage with a cap or tonneau cover?

[HANK1948]

I was wondering if anybody noticed better mileage with either, I have a buddy trying to decide to buy a cap or a tonneau and with one if any gives better mileage

[nedfunnell]

I was wondering if anybody noticed better mileage with either, I have a buddy trying to decide to buy a cap or a tonneau and with one if any gives better mileage

Have you tried removing or leaving down your tailgate?

[IUPAC]

Mythbusters tested two identical ford trucks, one with the tailgate down, one with it up. The truck that had the tailgate up, got better mileage.

[garysleeman]

I saw a mythbusters update this week where they tested the small ford pick-up with a tailgate net, fiberglass bed cover, tailgate up, tailgate down. I listed them in the order of best mileage achieved. I didn't watch the TV for the whole segment and was surprised by the net getting the best mileage. I assume they took the tailgate off completely for the net test.

[6.5 Detroit Diesel]

That's weird because with my tailgate off I got better milage on the highway.